An example of a system, based on stellar luminosity for predicting the location of the habitable zone around types of stars

In astronomy and astrobiology, habitable zone (more accurately, circumstellar habitable zone or CHZ) is the scientific term for the region around a star within which it is theoretically possible for a planet with sufficient atmospheric pressure to maintain liquid water on its surface.[1]1

"Habitable zone" is sometimes used more generally to denote various regions that are considered favorable to life in some way. One prominent example is the Galactic Habitable Zone, coined by Guillermo Gonzalez in 1995 (representing the distance of a planet from the galactic centre), based on the position of the Earth in the Milky Way. If different kinds of habitable zones are considered, their intersection is the region considered most likely to contain life.

The location of planets and natural satellites (moons) within its parent star's habitable zone (and a near circular orbit) is but one of many criteria for planetary habitability and it is theoretically possible for habitable planets to exist outside the habitable zone. The term "Goldilocks planet" is used for any planet that is located within the circumstellar habitable zone (CHZ)[4][5] although when used in the context of planetary habitability the term implies terrestrial planets with conditions roughly comparable to those of Earth (i.e. an Earth analog). The name originates from the story of Goldilocks and the Three Bears, in which a little girl chooses from sets of three items, ignoring the ones that are too extreme (large or small, hot or cold, etc.), and settling on the one in the middle, which is "just right". Likewise, a planet following this Goldilocks Principle is one neither too close nor too far from a star to rule out liquid water on its surface.

Dozens of planets have been confirmed in the habitable zone, though most found to date are significantly larger than the Earth, possibly due to sampling bias due to larger planets currently being more easily observed. The Kepler spacecraft has identified a further 54 candidates and current estimates indicate "at least 500 million" such planets in the Milky Way.[6]

Habitable zones, however, are not stable. Over the life of a star, the nature of the zone moves and changes.[7] Astronomical objects located in the zone are typically close in proximity to their parent star and as such are more exposed to adverse effects such as damaging tidal forces and solar flares. Combined with galactic habitability, these and many other exclusionary factors reinforce a contrasting theory of interstellar "dead zones" where life cannot exist, supporting the Rare Earth hypothesis.

Some planetary scientists have suggested habitable zone theory may prove limiting in scope and overly simplistic. There is growing support for equivalent zones around stars where other solvent compounds (such as ammonia and methane) could exist in stable liquid forms. Astrobiologists theorise these environments could be conducive to alternative biochemistry.[8] Additionally there is probably an abundance of potential habitats outside of the habitable zone within subsurface oceans of extraterrestrial liquid water. It may follow for oceans consisting of ammonia or methane.[9]

History

The concept of what is now widely known as the habitable zone originates in the 1950s. Two publications referring to the concept were written at about the same time. Hubertus Strughold wrote "The Green and the Red Planet: A Physiological Study of the possibility of Life on Mars" in which he used the term "ecosphere" and referred to "zones" in which life could exist.[2] In the same year, Harlow Shapley wrote the "Liquid Water Belt" which described the same theory in further scientific detail. Both stressed the importance of liquid water to life.[10] In 1955 Strughold wrote a follow-up called "Ecosphere of the Sun".[11] Chinese-American astrophysicist Su-Shu Huang extended the debate in 1959 with "Life-Supporting Regions in the Vicinity of Binary Systems" proposing that life zones were rare due to the orbital instabilities of habitable zones in common multistar systems.[12]

Habitable zone theory was further developed in 1964 by Stephen H. Dole in "Habitable Planets for Man" and then popularised by science fiction writer Isaac Asimov by capturing the imagination exploring possibilities of space colonization of other planetary systems.[13] Dole estimated the number of habitable planets in the Milky Way to be about 600 million.[14]

By the 1970s, Michael H. Hart's 1979 paper "Atmospheric Evolution, the Drake Equation and DNA: Sparse Life in an Infinite Universe" outlined the first evolutionary model for a habitable zone and his pessimistic conclusions on the distribution of extraterrestrial life fuelled the Rare Earth hypothesis.

Circumstellar habitable zone

Prediction of the extent of the Solar System's habitable zone indicated by two dimensional orbits of the inner Solar System bodies. The orbital region in which Earth could possess liquid water is indicated in green, with the core habitable zone indicated in dark green, along with the maximum (light green) indicating an extended habitable zone.

Beyond the outer edge of the habitable zone, a planet will be too cold to sustain liquid water on its surface. Any water present will freeze. A planet closer to its star than the inner edge of the habitable zone will be too hot. Any water present will boil away or be lost into space entirely. Liquid water is considered important because carbon compounds dissolved in water form the basis of all earthly life, so watery planets are good candidates to support similar carbon-based biochemistries.

Theoretical determinations of the habitable zone are based on empirical observation of the habitability of the Earth and its orbit within Solar System. Various complications must be taken into account, such as the greenhouse effect and changing albedo due to clouds.

With the discovery of large Super-Earth type planets, the concept of an Extended Habitable Zone has been adopted. This concept assumes that larger terrestrial planets could hold on to thicker atmospheres which could theoretically provide sufficient warming and pressure to maintain water at a greater distance from the parent star.

Solar System estimates

Estimates for the habitable zone within the Solar System range from 0.725 to 3.0 astronomical units based on various scientific models.

Estimation of the Solar System's habitable zone is made difficult due to a number of factors. Although the aphelion of planet Venus and the complete orbits of the Moon, the planet Mars and dwarf planet Ceres are within the habitable zone, the varying atmospheric pressures of these planets, rather than the habitable zone, determines their potential for surface water. In the case of Venus, the atmospheric pressure is far too high, and a runaway greenhouse effect raises the surface temperature massively, and in the case of Mars, the atmospheric pressure is too low, although Seasonal flows on warm Martian slopes have not yet been ruled out. For the Moon and Ceres atmosphere is virtually nonexistent, and therefore, surface liquid water cannot exist on these worlds.

Most estimates therefore are inferred on the effect that repositioned orbit would have on the habitability of Earth or Venus, therefore the habitable zone is based on calculations based on similar sizes and atmospheric pressures. According to extended habitable zone theory, a planet with a more dense atmosphere than Earth (or larger and more massive) orbiting in the extended habitable zone (possible like Gliese 667 Cd or Gliese 581 d) might theoretically possess liquid water.

Extrasolar extrapolation

Astronomers use apparent magnitude, luminosity and stellar flux along with the inverse square law to calculate habitable zones for stars. The "center" of the HZ is defined as the distance that an exoplanet would have to be from its parent star in order to receive the right amount of energy from the star to maintain liquid water. For example, a star with 25% of the luminosity of the Sun will have a CHZ centered at about 0.50 AU, while a star with twice the Sun's luminosity will have a CHZ centered at about 1.4 AU.

Other Habitability Considerations

Habitable zone theory suggests that it is possible for a planetary mass object in the habitable zone to sustain liquid water on its surface. However it does not follow that liquid water must exist on the surface of such a planet. Some of the following considerations relate to climate, rather than temperature, including nature of the star and the orbit of the planet can work for or against the presence of surface water on the planet. Along with these basic considerations, there are many additional criteria for planetary habitability.

Another way that water is theorised to originate is if an icy planetary mass body moves into a habitable zone causing its ice to melt and form surface water.

Atmospheric conditions

Thin atmosphere and resulting low temperatures are responsible for Mars dry surface. Water on Mars' surface instantly freezes and sublimates.

The habitable zone presumes that a planet has sufficient atmospheric pressure for surface water. In order for this to occur, the planet must either be of sufficient mass and gravity to for the required atmospheric pressure or have some source for the atmosphere to be continually replenished.

If the gravity is too low, then the planet will be less likely to retain sufficient atmospheric pressure for surface water and any remaining water would sublimate and more likely reach escape velocity and may be lost to space (as is thought to have occurred on Mars). If gravity is too high it could compress water to the point where it maintains a solid state regardless of the temperature.

Atmospheres are also known to regulate the temperature of a planet and as such its potential to sustain liquid water by contributing to a greenhouse effect and planetary albedo which are thought by some to be capable of causing runaway warming (as is thought to have occurred on Venus) and runaway cooling (as is thought to have occurred on Earth during Snowball Earth episodes). Atmospheric replenishing occurs on Earth via volcanism and processes such as the carbon cycle and biological processes. Other processes have been observed on other planets, such as exchange of atmospheric materials between Enceladus and Saturn through geysers as well as between Io and the other moons of Jupiter. Other theoretical processes include outgassing and cryovolcanism.

Planetary orbit

The orbit of the Earth and other planets within the Solar System is roughly circular. Earth's orbit allows its temperature to remain stable, near the triple point of water. Such an orbit is known as a 'continuously habitable orbit' However many exoplanets have been found with eccentric orbits, some of which cause them to spend some of their orbit outside of the zone. A notable example is 16 Cygni Bb. Venus is also known to spend only some of its orbit on the inside edge of the habitable zone. Temperature instability would cause water on such planets and their moons, should it exist, to likely go through extreme seasonal sublimation and deposition cycles. Standing bodies or surface water would be unstable and transient. It is currently unknown as to whether life is capable of adapting to such extreme cycles, should it be even capable of starting in the first place.

Planetary orbits have been known to change over a long period of time through a process of planetary migration. Icy planets and satellites migrating inward toward a star may become more habitable over time as the state of ices change to liquid and such planets may become ocean planets completely covered in water and lacking a solid surface.

Effect of Space Weather

Natural defenses against space weather, such as the magnetosphere depicted in this artistic rendition may be required for planets to sustain surface water for prolonged periods.

Space weather, in particular stellar radiation and stellar variation can significantly affect the ability of planets within the habitable zone to retain surface water. Venus and Mars are examples of planets that may have experienced significant and relatively rapid loss of surface water. Atmospheric escape can be caused by interaction with the stellar wind which can contribute to loss of pressure required to sustain water. In addition Photodissociation can convert atmospheric water into lighter gases. The two effects could combine to completely remove any hydrosphere from a planet.

A planet may require an intrinsic defense mechanism against the effects of space weather, such as a combination of magnetospheres, atmosphere, geological and geophysical cycles to sustain stable bodies water on the surface. The Earth, for instance, possesses such a combination of defenses.

Effect of Tidal forces

Stars smaller than the Sun have liquid-water zones much closer to the star and planets orbiting within their habitable zones would likely experience larger tides that could remove axial tilt, resulting in a lack of seasons. This could lead to much colder poles and a much hotter equator, and over time the planet's water may eventually be boiled away. Degrees of tidal locking could cause one-half of the planet to permanently face the star and the other half to be permanently frozen.[29] Alternatively, the day could resonate with the year causing prolonged periods of sublimation and deposition. An extrasolar moon orbiting a gas giant in the habitable zone may experience a more stable climate for water. Being locked to a planet, which does not radiate substantial energy, as opposed to a star, would allow starlight to reach nearly all of the surface as the moon orbits its primary. Such a satellite may be classed as a habitable moon at least in the context of being capable of sustaining stable bodies of liquid water on its surface.

Effect of Stellar Evolution

An artist's depiction of the Sun entering its red giant phase viewed from Earth. All life on Earth is extinct at this phase.

Over the life of a star, the nature of the zone moves and changes.[30] Stellar evolution can cause massive climate change over a period of millions of years, taking a planet in or out of the habitable zone. For instance, stars undergoing expansion and increasing in temperature may warm a planetary body, releasing gas and melting ices and creating bodies of surface water. The life of the habitable zone depends on the type of parent star, occurring faster or more slowly. Earth, for example is predicted to exit the Sun's habitable zone within a billion years as the Sun phases into a Red giant.

Habitable zones may remain stable for much longer around starts that are not main sequence. In the case of Red Dwarfs, the habitable zone may remain stable for billions of years.

Galactic habitable zone

The notion that the location of a planetary system within a galaxy must also be favorable to the development of life has led to the concept of a Galactic Habitable Zone (GHZ), developed in 1995 by Guillermo Gonzalez[31][32] although the concept has been challenged.[33]

Planetary habitability theory suggests star systems favorable to life should be located close enough to the galactic center for sufficient levels of heavy elements to form rocky (terrestrial) planets. This may not preclude life existing on gas giants or gaseous planets[34] which may be more common elsewhere, however life on gas giants (like Jupiter and Saturn) is currently considered less likely.[35] On the other hand, carbon-based life may be safer further away from the galactic center, where harmful high-frequency radiation is more intense.

Most of the stars in the galactic center are old, unstable, dying stars, meaning few or no stars form in the galactic center.[36] Some types of spiral galaxies in later time periods have been depleted of gas and dust in regions near to the galactic center, resulting in minimal new star formation in those parts of the galaxy. It is unlikely that terrestrial planets can form in the galactic center if stars cannot form there, because terrestrial planets form from the same types of nebulae as stars.

In our galaxy (the Milky Way), the GHZ is currently believed to be a slowly expanding region approximately 25,000 light years (8 kiloparsecs) from the galactic core and some 6,000 light years (2 kiloparsecs) in width, containing stars roughly 4 billion to 8 billion years old. Other galaxies differ in their compositions, and may have a larger or smaller GHZ – or none at all.

In 2008, a team of scientists described extensive computer simulations in the Astrophysical Journal[37] that show that, at least in galaxies similar to the Milky Way, stars such as the Sun can migrate great distances, thus challenging the notion that parts of these galaxies are more conducive to supporting life than other areas.[38][39]

Finding habitable zone planets and moons

Goldilocks planets are of key interest to researchers looking either for existing (and possibly intelligent) life or for future homes for the human race.[40]

The Drake equation, which attempts to estimate the likelihood of non-terrestrial intelligent life, incorporates a factor (ne) for the average number of life-supporting planets in a star system with planets. The discovery of extrasolar Goldilocks planets helps to refine estimates for this figure. Very low estimates would contribute to the Rare Earth hypothesis, which posits that a series of extremely unlikely events and conditions led to the rise of life on Earth. High estimates would reinforce the Copernicanmediocrity principle, in that large numbers of Goldilocks planets would imply that Earth is not especially exceptional.

Finding Earth-sized Goldilocks planets is a key part of the Kepler Mission, which uses a space telescope (launched on 7 March 2009 UTC) to survey and compile the characteristics of habitable-zone planets.[41] As of April 2011, Kepler has discovered 1,235 possible planets, with 54 of those candidates located within the Goldilocks zone.[42]

Discoveries in the zone

The majority of planets within our planet hunting neighbourhood are located within the GHZ, therefore the search for "habitable" planets has focused on data indicating a planet's position in the Goldilocks zone. The majority of these planets found have been gas giants, however more recently smaller Super-Earths and possible terrestrial planets have been detected in the zone.

Early discoveries: Gas giants and potential moons

Artist's impression of Upsilon Andromedae d, portrayed as a class II planet with water vapor clouds, as seen from a hypothetical large moon with surface liquid water

Although the extrasolar planet 70 Virginis b (discovered in 1996) was initially nicknamed "Goldilocks" because it was thought to be within the star's CHZ, it is now believed to be closer to its star, making it far too warm to be "just right" for life, instead being analogous to Venus.[43]

16 Cygni Bb (discovered in 1996) is a large gas giant with an eccentric orbit, found to spend some of its time inside the habitable zone. However the orbit means it would experience extreme seasonal effects. Despite this, simulations suggest an Earth-like moon would be able to support liquid water at its surface over the course of a year.[44]

Gliese 876 b (discovered in 1998) and Gliese 876 c (discovered in 2001) are both gas giants discovered in the habitable zone around Gliese 876. Although thought not to be watery, both may have habitable moons.[45]

Upsilon Andromedae d (discovered in 1999) is another gas giant discovered in the habitable zone considered large enough for the possibility of water clouds and watery moons.[46]

HD 28185 b (discovered April 4, 2001) is a gas giant found to orbit entirely within its star's habitable zone[47][48] and has a low orbital eccentricity, comparable to that of Mars in the Solar System.[49] Tidal interactions suggest that HD 28185 b could harbor Earth-mass satellites in orbit around it for many billions of years.[50] Such moons, if they exist, may be able to provide a habitable environment, though it is unclear whether such satellites would form in the first place.[51]

55 Cancri f (discovered in 2005), a Jupiter like gas giant exoplanet, orbits and also resides within the yellow dwarf star companion of 55 Cancri binary star systems habitable zone.[52] While conditions upon this massive and dense planet are not conducive to the formation of water or for that matter biological life as we know it, the potential exists for a system of moons to be orbiting the planet and thus transiting through this zone and being conducive for biological development.

Recent breakthroughs: Super-Earths and Earth-sized planets

The Gliese 581 system (first discovered in 2005) has a set of Super-Earths in a similar configuration to the inner Solar System. The third planet, planet c (discovered in 2007), is expected to be analogous to Venus's position (slightly too close), the fourth planet g (unconfirmed as of Oct. 2010) to the Earth/Goldilocks position, and the fifth planet d (discovered in 2007) to the Mars position. Planet d may be too cold, but unlike Mars, it is several times more massive than Earth and may have a dense atmosphere to retain heat. One caveat with this system is that it orbits a red dwarf, probably resulting in most of the issues regarding habitability of red dwarf systems, such as all the planets likely being tidally locked to the star.

On February 2, 2011, the Kepler Space Observatory Mission team released a list of 1235 extrasolar planet candidates, including 54 that may be in the "habitable zone".[53][54][55][56] Six candidates (KOI 326.01, KOI 701.03, KOI 268.01, KOI 1026.01, KOI 854.01, KOI 70.03) in the "Habitable Zone" are listed as smaller than twice the size of Earth,[56] although the one which got the most attention as "Earth-size" (KOI 326.01) turns out to be in fact much larger.[57] A September 2011 study by Muirhead et al. reports that a re-calibration of estimated radii and effective temperatures of several dwarf stars in the Kepler sample yields six additional Earth-sized candidates within the habitable zones of their stars: KOI 463.01, KOI 1422.02, KOI 947.01, KOI 812.03, KOI 448.02, KOI 1361.01.[58] Based on these latest Kepler findings, astronomer Seth Shostak estimates that "within a thousand light-years of Earth" there are "at least 30,000 of these habitable worlds".[59] Also based on the findings, the Kepler Team estimates "at least 50 billion planets in the Milky Way" of which "at least 500 million" are in the habitable zone.[6]

HD 69830 d (discovered in 2006) could be a rock-like or gas giant, 17 times the mass of Earth. [60]

HD 85512 b (discovered in 2011) is believed to be an Earth-like planet in the HD 85512 system and declared one of the best candidates to date in terms of habitability.[61]

A diagram comparing size (artists impression) and orbital position of planet Kepler-22b within Sun-like star Kepler 22's habitable zone and that of the Earth in the Solar System

Kepler 22 b, confirmed December 5, 2011.,[62] one of the first likely terrestrial planets detected in the habitable zone of a Sun-like main-sequence star (Kepler 22) using the transit method. Kepler 22 b is a super-Earth (2.4 times the size of Earth).

Gliese 667 Cc discovered in 2011 but announced in 2012[63] is a Super-Earth or gas giant in the Gliese 667 system found to be in an "extended habitable zone" and also one of the closest at 22.1 ly from Earth.

Gliese 163 c, discovered in September 2012 in orbit around the red dwarf Gliese 163[64] 49 ly from Earth, with 6.9 Earth masses and 1.8 to 2.4 Earth radii is expected to be on the hot side, receiving 40% more light from its star than Earth and with surface temperatures of at least 60C, though still has strong potential to possess surface water.[65][66][67]

Tau Ceti e and Tau Ceti f discovered in December 2012 have been found in the habitable zone around Tau Ceti, a Solar analog just 12ly away.[69] While larger than Earth, these are among the least massive planets found to date orbiting in the zone.

SETI

Habitable zones are used in the Search for Extra-Terrestrial Intelligence (SETI). This relies on the presumption intelligent extraterrestrial life, if it exists, is most likely to be found on planets within them. Habitable zone theory has been incorporated into recent applications of the Drake equation to estimate the number of intelligent races in the Milky Way.

For Active SETI, habitable zones are used as a means of selecting target stars for the transmission of interstellar radio messages (IRMs). In passive SETI, it is used to shortlist targets for sourcing non-natural radio emissions. The Allen Telescope Array is being used by the SETI Institute, using a list of candidate habitable planets discovered by the Kepler Space Telescope.[70] The Robert C. Byrd Green Bank Telescope, the largest fully steerable telescope on the planet, is being used by astronomers at the University of California, Berkeley to conduct a search for artificial radio emissions from habitable planets, including those identified by the Kepler mission.[71]

Criticism

The concept of a habitable zone is criticized by Ian Stewart and Jack Cohen in their book Evolving the Alien, for two reasons: the first is that the hypothesis assumes alien life has the same requirements as terrestrial life; the second is that, even assuming this, other circumstances may result in suitable planets outside the "habitable zone". For instance, Jupiter's moon Europa is thought to have a subsurface ocean with an environment similar to the deep oceans of Earth. The existence of extremophiles (such as the tardigrades) on Earth makes life on Europa seem more plausible, despite the fact that Europa is not in the presumed CHZ. Astronomer Carl Sagan believed that life was also possible on the gas giants, such as Jupiter itself. A discovery of any form of life in such an environment would expose these hypothetical restrictions as too conservative. Life can evolve to tolerate extreme conditions when the relevant selection pressures dictate, and thus it is not necessary for them to be "just right".[72]

Differing levels of volcanic activity, lunar effects, planetary mass, and even radioactive decay may affect the radiation and heat levels acting on a planet to modify conditions supporting life. And while it is likely that Earth life could adapt to an environment like Europa's, it is far less likely for life to develop there in the first place, or to move there and adapt without advanced technology. Therefore, a planet that has moved away from a habitable zone is more likely to have life than one that has moved into it.[73]

Just as different levels of atmospheric pressure can affect water remaining in liquid state, the presence of dissolved compounds, such as salts or ammonia (a powerful antifreeze) can lower the freezing point of water. Earth's oceans are saline, which helps prevent them freezing. Greenhouse effects, solvents in water or a combination of these factors could potentially enable planets with certain conditions to sustain surface water. For example, briny water is proposed as an explanation for Seasonal flows on warm Martian slopes.